Make Google Docs file read only / lock / protect

makeGoogleDocFileReadyOnly

// 1. Open your document on Google.
// 2. Go to Tools > Script Editor.
// 3. Copy the following script and save the script.
// 4. Go back to the document.
// 5. Go to the new Custom menu > Make file read only
// 6. To unlock, there will be an Unlock button on the UI (provided by Google)
// 
// Inspired by https://www.labnol.org/code/read-only-google-drive-file-201011

const makeGoogleDocFileReadyOnly = () => {
  const fileId = DocumentApp.getActiveDocument().getId();
  UrlFetchApp.fetch(`https://www.googleapis.com/drive/v3/files/${fileId}`, {
    method: 'PATCH',
    contentType: 'application/json',
    headers: {
      Authorization: `Bearer ${ScriptApp.getOAuthToken()}`,
    },
    payload: JSON.stringify({
      contentRestrictions: [
        {
          readOnly: true,
          reason: 'Prevent accidental editing',
        },
      ],
    }),
  });
};

function onOpen() {
  var ui = DocumentApp.getUi();
  ui.createMenu('Custom Menu')
      .addItem('Make file read only', 'makeFileReadyOnlyMenuItem')
      .addToUi();
}

function makeFileReadyOnlyMenuItem() {
  makeGoogleDocFileReadyOnly()
  DocumentApp.getUi() 
     .alert('You clicked the first menu item!');
}

do I need to run the script? there is error by the way

TypeError: Cannot read properties of null (reading 'getId')

@thanhtam23 Step 5 of the instructions (top of file) is where you run the script , i.e., by clicking the menu item.

It works fine for me.